Technologies for deodorization, sterilization, and toxic gas decomposition based on a plasma developed by high-voltage pulse discharges have recently been put to use. To generate such a plasma, a high-voltage pulse generating circuit capable of supplying extremely narrow pulses of a high voltage is required.
As shown in FIG. 10 of the accompanying drawings, a conventional high-voltage pulse generating circuit 100 has a charger device 102 for generating a high DC voltage which is substantially equal to the peak value of a high-voltage pulse, a capacitor 104 for being charged to the high DC voltage generated by the charger device 102, a switch 108 comprising a plurality of semiconductor devices 106 such as SI (Static Induction) thyristors or the like which are connected in series to provide a high withstand voltage, and a load 110 that is supplied with a high-voltage pulse by high-speed switching operation of the switch 108 under the high DC voltage charged in the capacitor 104 (see, for example, Patent Document 1).
The switch 108 has a plurality of gate drive circuits 112 connected to the respective semiconductor devices 106 to turn on the semiconductor devices 106, and a plurality of balancer resistors 114 connected in parallel to the respective semiconductor devices 106. The balancer resistors 114 serve to reduce any unbalanced differences between the voltages applied across the respective semiconductor devices 106 due to impedance variations caused when the semiconductor devices 106 are rendered nonconductive.
Specifically, the high-voltage pulse generating circuit 100 has a multiple-series-connected circuit 116 of the semiconductor devices 106 and the balancer resistors 114 which are connected in series to the load 110.
FIG. 11 of the accompanying drawings shows a proposed high-voltage pulse generating circuit 118. In the proposed high-voltage pulse generating circuit 118, when a semiconductor switch 126 is turned on, a current flows from a DC power supply 120 (having a power supply voltage E) to a resistor 136 (having a resistance R) to the one-turn primary windings of respective magnetic cores 128 to the semiconductor switch 126 to the DC power supply 120, the current having a magnitude represented substantially by E/R.
At this time, because of the magnetic cores 128 operating as a transformer, the same current flows through the one-turn secondary windings of respective magnetic cores 128 via the gates and cathodes of semiconductor devices 134. Therefore, all the semiconductor devices 134 are simultaneously turned on (see, for example, Non-patent Document 1).
Since the semiconductor devices 134 connected in series to the semiconductor switch 126 are rendered conductive, a voltage which is substantially the same as the power supply voltage E is applied to an inductor 138. As a result, a current IL flowing through the inductor 138 increases linearly, storing electromagnetic energy in the inductor 138.
The current IL flowing through the inductor 138 increases until electromagnetic energy is stored up to a desired level in the inductor 138. When the semiconductor switch 126 is then turned off, since the path of the current IL flowing through the inductor 138 is cut off, an induced voltage of opposite polarity is generated due to the stored electromagnetic energy in the inductor 138.
As a consequence, the diode 140 is rendered conductive, allowing a current to flow continuously from the inductor 138 to the semiconductor devices 134, the primary windings of the respective magnetic cores 128 to the diode 140 to the inductor 138. At this time, a current of the same magnitude also flows through the secondary windings of the magnetic cores 128.
Thus, the current flowing into the anodes of the semiconductor devices 134 flows in its entirety to the gates thereof, with no current flowing to the cathodes thereof. The current flows until the electric charges stored in the semiconductor devices 134 are discharged. Since no large voltage drop is caused in the current path and this state continues for an extremely short period of time, reduction in the current IL flowing through the inductor 138 is small, and reduction in the stored electromagnetic energy in the inductor 138 is also small.
As the electric charges stored in the semiconductor devices 134 are discharged, the semiconductor devices 134 are turned off, with a depletion layer being quickly developed therein. Since the inductor current is charged with a small electric capacity, the voltage between the anode and cathode of each of the semiconductor devices 134 rises sharply. Therefore, the voltage across the inductor 138 increases quickly, and the current IL flowing through the inductor 138 decreases quickly. Stated otherwise, the electromagnetic energy in the inductor 138 is shifted into an electrostatic energy in the capacitance between the anode and cathode of each of the semiconductor devices 134. Since the voltage across the inductor 138 is also applied to a load 142, the electromagnetic energy in the inductor 138 and the electrostatic energy in the capacitance between the anode and cathode of each of the semiconductor devices 134 are consumed by the load 142 while the electromagnetic energy is being shifted into the electrostatic energy.
With the high-voltage pulse generating circuit 118, the DC power supply 120 may generate a low voltage and the semiconductor devices 134 may be turned on and off only by currents flowing through the secondary windings of the magnetic cores 128. Consequently, the high-voltage pulse generating circuit 118 requires no gate drive circuits and is relatively simple.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-44965
Non-patent document 1: The Institute of Electrical Engineers of Japan, Plasma Science and Technology, Lecture No. PST-02-16 (FIG. 1)